Many articles, for example food, electronic devices or pharmaceuticals, are very sensitive to moisture and/or oxidizing agents. Many of these products rapidly degrade upon exposure to water, oxidizing agents or other gases or liquids. Polymeric substrates, such as polymeric foils, are often used to package these products. These foils frequently exhibit a permeability for water vapor and for oxidizing agents in the range of more than 1 g/(m2day). This high degree of permeability is unacceptable for most of the products packaged by polymeric foils.
One packaging application that uses polymeric substances is the packaging of organic electroluminescent devices (OLEDs). An OLED device includes a functional stack formed on a substrate. The functional stack has at least one organic functional layer sandwiched between two conductive layers. The conductive layers serve as electrodes (cathode and anode). When a voltage is applied to the electrodes, charge carriers are injected through these electrodes into the functional layer and upon recombination of the charge carriers visible radiation can be emitted (electroluminescence). This functional stack of the OLED tends to be very sensitive to moisture and oxidizing agents, which can cause, for example, oxidation of the metals of the electrodes or deterioration of the organic functional layers.
Due to the intrinsic properties of the organic functional layers, flexible OLED devices can be built up on flexible substrates, for example, polymeric substrates. For sufficient life-time of the OLEDs, polymeric substrates with a permeability for water or oxidizing agents below 10−6 g/(m2day) are desireable.
FIG. 1 depicts a conventional environmentally sensitive display device encapsulated by a barrier assembly having a ceramic barrier layer and a polymeric layer. A device 25 is arranged on a substrate 5. The device 25 is encapsulated by a barrier stack having a ceramic barrier layer 20 and a passive polymeric layer 15. Due to the high permeability of the polymeric layer 15 most of the barrier abilities of the barrier stack are attributed to the ceramic barrier layer 20.
Tightly controlled conditions for the deposition of this ceramic barrier layer are typically necessary for the ceramic barrier layer to have a low incidence of defects, such as pinholes, grain boundaries and shadowing effects. These defects can provide a continuous path for permeants to pass through the ceramic layer and the defects can therefore lead to a decreased ability of the ceramic barrier layer to function as a barrier. Often several thin ceramic layers are deposited on top of each other in order to enhance the barrier abilities of the ceramic barrier layers. This can lead to a complicated and therefore expensive production of flexible barrier layers.